1. Field of the Invention
The present invention relates generally to gate valves and, more particularly, relates to a specific type of gate valve referred to as a parallel expanding gate valve.
2. Description of the Prior Art
Gate valves are generally comprised of a body having a central axis aligned with inlet and outlet passages, and a space between the inlet and outlet passages in which a slide, or gate, may be moved perpendicular to the central axis to open and close the valve. In the closed position, flat surfaces on the gate typically seal against sealing rings which surround the fluid passage through the valve body.
Gate valves have been used for centuries to control the flow of a great variety of fluids. Often the fluid to be controlled by the gate valve is under pressure, in which case it has been found difficult to obtain a completely tight seal which will remain fluid tight after the valve has been in use for some time. Exemplary prior art gate valves and gate valve seats are disclosed in U.S. Pat. Nos. 4,471,943 and 4,625,942.
Frequently the sealing rings used in gate valves are composed of various types of resilient elastomeric materials. It can be appreciated that sealing rings having a large sealing surface area and having high compressibility against the gate sealing surfaces can be highly effective in forming a fluid-tight seal. However, these same qualities in the sealing rings also result in substantial wear of the elastomeric sealing rings due to the frictional movement of the gate. This high friction makes operation of the valve very difficult and requires a large and expensive valve operator. Various techniques have been developed to circumvent these problems, including providing very smooth surfaces to improve sealing and reduce friction, lubricating the sealing surfaces, maintaining close tolerances and accurate alignments, and pressure balancing against the seals using relief pressure ports. These techniques are often expensive and do not always provide the reliability, operating range, and long life desired for industrial gate valves. Additionally, the elastomeric materials useful for effecting seals are not always suitable for control of all types of fluids, and may have operating temperature range limitations.
Another approach to solving the problem of sealing in gate valves involves using expandable gates. In an expandable gate valve, the gate remains collapsed while moving between open and closed position, but expands when placed opposite the sealing rings to effect a fluid-tight seal. In this way it is possible, in general, to avoid the high friction which wears on the sealing rings and makes operation of the valve difficult. Parallel expanding gates apply a considerable axially directed force against the sealing rings so that it is possible to obtain a fluid tight seal using metal seals or metal seals having some type of protective coating but having virtually no resilience. For this reason, parallel expanding gate valves may be used with metal seals in applications having relatively wide temperature operating ranges and in controlling fluids which may have a tendency to deteriorate elastomeric materials.
One presently available parallel expanding gate valve has a gate which is composed of two segments adjacent to and substantially parallel with each other. The inner sides of each segment have mating inclined surfaces and the outer sides of each segment form sealing surfaces. An operating stem is attached to one of the segments, which may be called the gating segment, and a spring holds the segments together in a collapsed position while the gate is moving from the opened to the closed valve positions.
To close the above described parallel expanding gate valve, the operating stem pushes the gating segment downwards until the other segment, which is longer than the gating segment, reaches a lower stop. As the operating stem continues to press downwards on the gating segment, the two segments expand axially outward with respect to each other due to their inclined inner surfaces, and press their sealing surfaces against the seals to form a fluid-tight seal. Similarly, to open the valve, the operating stem pulls the gating segment until the longer segment reaches an upper stop. The gating segment is pulled further so that the gate expands due to the inclined surfaces and presses against the seals. An aperture through the gate is, at this point, now aligned with the passage through the bore to allow fluid flow through the valve.
Significant problems exist with this type of parallel expanding gate valve. While the gate is moving particles or debris in the fluid, often called fill, may collect in the valve body at the lower stop to effectively alter the stop point. Since the gate now stops at a point at which it was not designed to stop, the forces acting on the sealing surfaces which result from the inner inclined surfaces of the segments may no longer be as uniform as they were designed to be. The segments may press harder against one part of the seals than another part so that the seal may not be as tight as normally it would be. This will be especially true if there has been some wear on the seals which may have been caused by friction forces which occur during the continued downward movement of the gate while the segments are expanding. Also, due to the sometimes very high downward forces acting on the longer segment at the lower stop or stop wall, some gouging, digging or general deformation may occur at the stopping edge of the longer segment or at the stop wall. This occurrence also effectively alters the correct stop point and tends to create a similar unequal distribution of force against the sealing rings, as previously described.
In this prior art parallel expanding gate valve, because the segments press directly against each other, any problem that occurs to one segment will have a direct effect on the other segment. If, for instance, one segment is somehow bent or tilted and loses its seal, the other segment will be affected so that its seal may also be lost.
There is also a problem with this parallel expanding gate valve relating to the outward movement of the gates with respect to each other. As the segments are expanding outwardly away from each other, to prevent bending of the operating shaft, which may be attached with threads to the gating segment, generally the longer segment is the segment designed to be the segment which moves outwardly and the gating segment preferably remains substantially stationary with respect to the axis of the operating shaft. However, because the longer segment is also pressing downwards against the stop wall, friction problems may develop between the stopping edge of the longer segment and the stop wall as the longer segment slides outward. These problems can result in gouging or sticking of the long segment during its outward movement. The collection of foreign particles in the body exacerbate the sticking and gouging. If, due to fill or gouging, the bottom edge of the longer segment is prevented from completing its horizontal movement, some bending or tilting may occur which, as explained previously, could affect the seal of both segments. Note that with this parallel expanding gate some bending of the operating stem is inevitable because the gating segment to which the operating stem is secured must also expand outward to some degree to effect a seal.
While the gating segment is designed to remain substantially in line with the operating stem, during the expanding process it continues to move along the operating stem axis to effect expansion. This means that wear will occur primarily on one seal only due to the considerable friction created between the gating segment and seal as it moves until it is effectively stopped by friction forces from moving further. This asymmetrical wear on one seal may affect the tightness of the other seal if there is any tilting of the gating segment due to wear on the gating segment seal because of the direct relationship between the gating segment and the longer segment.
Another related problem which may arise in a presently available parallel expanding valve is that the gating segment may "bottom out" or hit the stop wall prior to the occurrence of sufficient expansion of the segments to effect sufficiently tight seals. This can occur due to wear on the seals or sealing surface or due to a possible change in the stop point as previously discussed. This problem becomes more pronounced as the angles of the inner surfaces (with respect to a line perpendicular to the central axis) become small. Smaller angles have the advantage of generally providing a greater expansion force but the disadvantage of requiring more travel of the stem and friction between moving parts.
It will be appreciated that when problems occur which are the result of damage of the stop wall, which is typically an integral part of the valve body, repair will then probably require changing out the entire body of the valve.
In summary, the presently existing parallel expanding gate valves are susceptible to sealing problems resulting from normal fill in the valve body, gouging or distortion at the stop wall and on the stopping edge of the longer segment, possible bending of the longer segment and/or the operating stem during expansion of the gate, and wear on the seals or sealing surfaces.
Consequently, a need exists for improvements in parallel expanding gate valves which will result in greater reliability and dependability of operation.